Hydrostatic transmission assemblies are widely used in a variety of applications ranging from small to heavy load applications. The hydraulic motors in hydrostatic transmission assemblies are used to cause rotational movement in systems such as, e.g., hydraulic systems. Often, one or more hydrostatic transmission assemblies are included in the system which can be subject to frequent loads in a harsh working environment, e.g., in the hydraulic systems of industrial machines such as excavators, front-end loaders, and cranes. Typically, in such conventional machines, the actuator components include numerous parts such as a hydraulic motor, a central hydraulic pump, a motor to drive the pump, a fluid reservoir and appropriate valves that are all operatively connected to perform work on a load, e.g., rotating an excavator blade or a grader blade on an excavator.
The motor drives the hydraulic pump to provide pressurized fluid from the fluid reservoir to the hydraulic motor, which in turn causes the shaft of the hydraulic motor to move the load that is attached to the hydraulic motor. When the fluid exits the outlet of the hydraulic motor, the fluid is sent back to the fluid reservoir. To control the flow, the hydraulic system can include a variable-displacement hydraulic pump and/or include a hydraulic pump in combination with a directional flow control valve (or another type of flow control device). In these types of systems, the motor that drives the hydraulic pump is often run at constant speed and the directional flow control valve (or other flow device) controls the flow rate of the hydraulic fluid. The directional flow control valve can also provide the appropriate porting to the hydraulic motor to rotate the motor. The pump is kept at a constant speed because the inertia of the hydraulic pump in the above-described industrial applications makes it impractical to vary the speed of the hydraulic pump to precisely control the flow or pressure in the system. That is, the prior art pumps in such industrial machines are not very responsive to changes in flow and pressure demand. Thus, the hydraulic pump is run at a constant speed, e.g., full speed, to ensure that there is always adequate fluid pressure at the flow control devices. However, running the hydraulic pump at full speed or at some other constant speed is inefficient as it does not take into account the true energy input requirements of the system. For example, the pump will run at full speed even when the system load is only at 50%. In addition, along with being inefficient, operating the pump at full speed will increase the temperature of the hydraulic fluid. Further, the flow control devices in these systems typically use hydraulic controls to operate, which are complex and can require additional hydraulic fluid in the system.
Because of the complexity of the hydraulic circuits and controls, the hydraulic systems described above are typically open-loop in that the pump draws the hydraulic fluid from a large fluid reservoir and the hydraulic fluid is sent back to the reservoir after performing work on the hydraulic actuator and controls. That is, the output hydraulic fluid from the hydraulic actuator and the hydraulic controls is not sent directly to the inlet of the pump as in closed-loop systems, which tend to be for simple systems where the risk of pump cavitation is minimal. The open-loop system helps to prevent cavitation by ensuring that there always an adequate supply of fluid for the pump and the relatively large fluid reservoir in these systems helps maintain the temperature of the hydraulic fluid at a reasonable level. However, the open-loop system further adds to the inefficiency of the system because the fluid resistance of the system is increased with the fluid reservoir. In addition, the various components in an open-loop system are often located spaced apart from one another. To interconnect these parts, various additional components like connecting shafts, hoses, pipes, and/or fittings are used, which further adds to the complexity and resistance of the system. Accordingly, the above-described hydraulic systems can be relatively large, heavy and complex, and the components are susceptible to damage or degradation in the harsh working environments, thereby causing increased machine downtime and reduced reliability. Thus, known systems have undesirable drawbacks with respect to complexity and reliability of the systems.
Further limitation and disadvantages of conventional, traditional, and proposed approaches will become apparent to one skilled in the art, through comparison of such approaches with embodiments of the present invention as set forth in the remainder of the present disclosure with reference to the drawings.